Deuterated bipolar transistor and method of manufacture thereof

ABSTRACT

A bipolar transistor and a method of manufacturing the transistor. The transistor includes: (1) a substrate having a base region, an emitter region and a base-emitter junction between said base and emitter regions and (2) a substantial concentration of an isotope of hydrogen located in said biploar transistor.

This Application is a Divisional of prior application Ser. No.08/848,113 filed on Apr. 28, 1997, now U.S. Pat. No. 5,982,020, to IsikKizilyalli, et al. The above-listed Application is commonly assignedwith the present invention and is incorporated herein by reference as ifreproduced herein in its entirety under Rule 1.53(b).

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to a transistorfabrication and, more specifically, to a bipolar transistor in which thebase-emitter junction therein is exposed to deuterium while thetransistor is being fabricated and a method of manufacture of such atransistor.

BACKGROUND OF THE INVENTION

The use of silicon in devices, such as n-p-n bipolar transistors, iswell known. Equally well known is the time dependent degradation ofthese devices, which is caused by reverse-bias stress of theemitter-base junction of the n-p-n transistors. Reverse-bias stressresults in the degradation of the common-emitter current gain(H_(FE)=ΔI_(C)/ΔI_(B)). However, it is thought that the collectorcurrent (I_(C)) is not affected by the stress, but an increase in therecombination component of the base current at low V_(BE) has beenobserved. In BICMOS and BINMOS circuits this reverse-biasing will resultin a long term performance degradation and eventually circuit failure.It is believed that the nature of the damage mechanism is that hotcarriers generated in the reverse-biased base-emitter junction createinterface trap states on the base oxide by breaking silicon/hydrogen,often referred to as the hot carrier degradation effect. Morespecifically, it is believed that the base-oxide damage (interface trapgeneration) is caused by the interaction of hot electrons with theSi/SiO₂ interface. Electrons that are generated by band-to-band (fromthe valence band to the bottom of the conduction band) tunneling at thebase-emitter junction are subsequently accelerated (heated) by thejunction electric field. In most cases, the substrate, as well as otherstructures within the device, comprises silicon, and the defects arethought to be caused by dangling bonds (i.e., unsaturated silicon bonds)that introduce states in the energy gap, which remove charge carriers oradd unwanted charge carriers in the device, depending in part on theapplied bias. While dangling bonds occur primarily at surfaces orinterfaces in the device, they also are thought to occur in the bulkoxide. To alleviate the problems caused by such dangling bonds, ahydrogen passivation process has been adopted and has become awell-known and established practice in the fabrication of such devices.

In the hydrogen passivation process, it is thought that the defects thataffect the operation of semiconductor devices are removed when thehydrogen bonds with the silicon at the dangling bond sites. While thehydrogen passivation process eliminates the immediate problem associatedwith these dangling bonds, it does not eliminate degradation permanentlybecause the hydrogen atoms that are added by the passivation process canbe “desorbed” or removed from the previous dangling bond sites by thehot carrier effect.

A hot carrier is an electron or hole that has a high kinetic energy,which is imparted to it when voltages are applied to electrodes of thedevice. Under such operating conditions, the hydrogen atoms, which wereadded by the hydrogen passivation process, are knocked off by the hotelectrons. This hydrogen desorption results in aging or degradation ofthe device's performance. This hot carrier effect is particularly ofconcern with respect to smaller devices, such as a bipolar transistors.

Accordingly, what is needed in the art is a bipolar transistor deviceand a method of manufacture therefore that does experience the level ofefficiency degradation experienced by the devices that are passivatedwith conventional hydrogen passivation processes. The present inventionaddresses these needs.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, thepresent invention provides a bipolar transistor and a method ofmanufacturing the transistor. The transistor includes: (1) a substratehaving a base region, an emitter region and a base-emitter junctionbetween said base and emitter regions and (2) a substantialconcentration of an isotope of hydrogen located in the vicinity of thebase-emitter junction.

The present invention therefore introduces the broad concept ofemploying, in lieu of hydrogen, an isotope of hydrogen to passivatematerial in the base-emitter junction in a bipolar transistor. Forpurposes of the present invention, “substantial concentration” isdefined as a concentration of at least 10¹⁶ cm⁻³ of isotopic hydrogen.

In one embodiment of the present invention, the isotope is deuterium.The principles of the present invention may be applied to heavierisotopes of hydrogen including tritium and any later-discoveredisotopes.

In one embodiment of the present invention, the transistor furthercomprises a collector region in said substrate and a base-collectorjunction between said base and collector regions. Those who are skilledin the art may recognize benefits in deuterating the base-collectorjunction, as well, although such is not necessary to the presentinvention.

In another embodiment of the present invention, the base-emitterjunction is capable of transmitting reverse-bias electrical currents. Asmentioned above, reverse-bias currents can be injurious to bipolartransistors not having deuterated base-emitter junctions.

In one embodiment of the present invention, the emitter region is atleast partially composed of polysilicon that has a substantialconcentration of a hydrogen isotope located or incorporated therein.Those who are skilled in the art are familiar with polysiliconstructures that may be deposited on a substrate to form poles of atransistor.

In yet another embodiment of the present invention, the transistorfurther comprises a dielectric layer located over the life substrate. Inpreferred embodiments, the dielectric also has a substantialconcentration of a hydrogen isotope located or incorporated therein. Thepresent invention is fully compatible with current nonisotopic hydrogenpassivation techniques, which can work with dielectric layers.

In another embodiment of the present invention, the isotope iscovalently bonded to material in the base-emitter junction. Alternativebonding structures may exist, however. The present invention isindependent of the type of bond between the isotope and the material inthe base-emitter junction.

In yet another embodiment of the present invention, the substrate isgenerally planar and the isotope is concentrated in portions of a planarsurface of the substrate. Thus, the isotope may be patterned.

In another embodiment of the present invention, the substrate iscomposed at least partially of silicon. However, other conventional orlater-discovered substrate materials are within the scope of the presentinvention.

In one embodiment of the present invention, the transistor furthercomprises at least one electrical conductor that carries electricalcurrent to the transistor. Those who are skilled in the art willrecognize that the present invention allows, but does not require,electrical conductors to be associated with the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a schematic cross-sectional view of a bipolartransistor;

FIG. 2 illustrates a graph that shows hot carrier stress experimentsconducted on a transistor at peak substrate current conditions; and

FIG. 3 illustrates a transistor lifetime versus substrate current.

DETAILED DESCRIPTION

Referring initially FIG. 1, there is illustrated a schematic,cross-sectional view of a bipolar transistor device 10 of the presentinvention, which is preferably a n-p-n transistor. In advantageousembodiments, the bipolar transistor 10 is comprises a substrate 12 witha collector 14 formed over it. In preferred embodiments, the substrate12 is a p-type substrate and the collector is a n⁺-type that has an⁻-type layer 16 formed over it with a p-type base 18 formed over then⁻layer 16. The substrate 12, collector 14 and the above-mentionedsubsequent layers are formed by conventional processes. The substrate 12may comprise silicon, germanium, gallium arsenide or other presentlyknown or later-discovered materials that are suitable for themanufacture of such bipolar transistors. In one desirable embodiment,however, the substrate 12 is composed at least in part of silicon.

In a more detailed embodiment, the bipolar transistor 10 includes one ormore base dielectric structures 20. The base dielectric structure 20contains a substantial concentration of a hydrogen isotope and is,preferably, thermally grown in the presence of an isotopic steam ofhydrogen. For purposes of the present invention, the isotopic steamshould have as high a concentration of the hydrogen isotope as possible.In more advantageous embodiments, the nonisotopic hydrogen should notexceed 1 ppm within the steam. In one advantageous embodiment, theisotopic steam is deuterium (D₂O), however, the principles of thepresent invention may be applied to even heavier isotopes of hydrogen,including ionic forms of the various isotopes of hydrogen.

Alternatively, the base dielectric structure 20 may be chemicallydeposited from a gas mixture containing a substantial concentration of ahydrogen isotope, such as deuterium. For purposes of the presentinvention, “substantial concentration” is defined as a concentration ofat least about 10¹⁶ cm⁻³ of isotopic hydrogen. Representative examplesof such gases and gas mixtures include: deuterated silane and oxygen(SiD₄+O₂), deuterated silane and nitrous oxide (SiD₄+N₂O), deuteratedtetraethyl orthosilicate (TEOS, Si(OC₂D₅)₄), deuterated silane (SiD₄),deuterated nitride (ND₃) or deuterated dichlorosilane and nitrous oxide(SiCl₂D₂+N₂O). Other gas mixtures typically used to form variousstructures within the bipolar transistor 10 may also be used, with theexception that they contain a substantial concentration of isotopichydrogen. In advantageous embodiments, the ordinary or nonisotopichydrogen should not exceed 1 ppm within the gas mixture. The gas mixtureis injected into the deposition chamber and passivation is conducted attemperatures ranging from about 100° C. to about 1100° C. The pressureat which the passivation occurs may be either at, above or well belowatmospheric pressures, and the flow rate of the gaseous material willdepend on the equipment used for depositions. These conditions combine,to form a preferred formation rate that may range from about 0.01 nm toabout 100.0 nm per minute. However, in more desirable embodiments, theformation rate may range from about 1 nm to about 20 nm.

When the base dielectric structure 20 is passivated with the hydrogenisotope, it is believed that the dangling bond sites are occupied by thehydrogen isotope. It is further believed that this passivation greatlyreduces degradation within the bipolar transistor 10 because thedangling bond sites are no longer available to remove charge carriers oradd unwanted charge carriers in the device. Furthermore, the hydrogenisotope may form a bond with the substrate 12 that is harder to breakresulting in more reliable devices, optical or electrical. A suggestedexplanation why the bond is harder to break is that the hydrogen'sisotopes have a heavier mass than ordinary hydrogen, which makes it moredifficult to remove the isotope. Thus, the presence of the hydrogenisotope within the base dielectric structure 20 offers distinctadvantages over the devices of the prior art.

Also illustrated in FIG. 1 is an emitter structure 22 that has beendeposited, doped and etched using conventional processes. The emitterstructure 22 is typically positioned over the p-type base 18 andpreferably comprises polysilicon. A hydrogen isotope may be incorporatedinto this structure by gaseous deposition of a silicon materialcontaining a substantial concentration of a hydrogen isotope. Theprocess for forming the emitter structure 22 is well known, with theexception, of course of the use of a gaseous material containing asubstantial concentration of a hydrogen isotope. Representative examplesof such deposition gases could include deuterated silane (SiD₄). Inadvantageous embodiments, the ordinary or nonisotopic hydrogen does notexceed 1 ppm within the gas mixture. The temperature at which the gasmixture is injected into the deposition chamber is pressure dependentand may vary substantially. The pressure at which the passivation isconducted may be either at, above or well below atmospheric pressures.The rate of deposition may also vary, depending on the desired thicknessand uniformity of the layer. The conditions and processes for depositingthe isotopic hydrogen gas are known to those who are skilled in the art.Deposition conditions combine to form a preferred formation rate thatmay range from about 0.01 nm to about 10.0 nm per minute. However, inmore desirable embodiments, the formation rate may range from about 0.5nm to about 3 nm.

When the emitter structure 22 is passivated with the hydrogen isotope,it is believed that the dangling bond sites within it are occupied bythe hydrogen isotope as previously explained. This stronger hydrogenisotope/silicon bond provides transistors that are more robust and thathas a substantially lower rate of degradation.

The substrate 12, base dielectric structures 20 and emitter structure 22provide a resulting structure that is representative of a foundationallevel typically found in the early stages of bipolar transistorfabrication processes.

Subsequent to the formation of the emitter structure 22, the entirefoundation level may have a dielectric 24 formed over it to form afoundation for the next interconnect level. The dielectric 24 may beformed by conventional process with the exception that the gaseousmaterial, such as deuterated TEOS or deuterated silaane, must have asubstantial concentration of a hydrogen isotope therein. As seen fromFIG. 1, the present invention can be used to incorporate a hydrogenisotope into the various structures within the bipolar transistor 10, ifso desired. When so incorporated, the hydrogen isotope provides astructure having the above-described advantages associated therewith.

Alternatively, or in addition to the processes discussed above, thehydrogen isotope may be incorporated into the bipolar transistor eitherat any time during the manufacturing process or at the end of theprocess and just prior to the capping of the device. In suchembodiments, the hydrogen isotope is introduced at temperatures rangingfrom about 200° C. to about 1000° C. in a forming gas anneal step fortime periods ranging from about 10 minutes to 2 hours or more. Theforming gas may be, for example, a mixture of nitrogen and the hydrogenisotope, or a mixture of nonisotopic hydrogen and the hydrogen isotope.This annealing step is well known with the exception that a hydrogenisotope is present and is present within the gas mixture in asubstantial concentration as discussed above.

FIG. 2 is a graph that illustrates hot carrier stress experimentsconducted on transistors at peak substrate current conditions. Theinterface damage, caused by hot carriers, is observed by monitoring thechange in the linear transconductance (g_(m)) and threshold voltage(V_(th)) of the NMOS transistor. FIG. 3 also shows the V_(th)degradation as a function of stress time. As shown by the graph, thedegradation of the transistor passivated with hydrogen is significantlyhigher than the degradation observed for the device passivated withdeuterium. The threshold voltage for the deuterium passivated deviceincreases to only about 1.05 volts over a period of 10⁴ minutes whereasat that same period of time, the threshold voltage for the hydrogenpassivated device increases to 1.3 volts.

FIG. 3 shows NMOS transistor lifetime versus substrate current. Fromthis data it is evident that devices annealed in hydrogen isotopes, suchas deuterium, are much more robust under channel hot electron stress.The extrapolated transistor lifetimes are indicated using variousdegradation criteria. Given the similar structures between the NMOS andthe bipolar transistor devices covered by the present invention, it isapparent that the same results could be expected in bipolar transistordevices as appears in the NMOS devices.

From the foregoing it is readily seen that the present inventionprovides a bipolar transistor and a method of manufacturing thetransistor. The transistor includes: (1) a substrate having a baseregion, an emitter region and a base-emitter junction between said baseand emitter regions and (2) a substantial concentration of an isotope ofhydrogen located in the vicinty of the base-emitter junction. Thepresent invention, therefore, introduces the broad concept of employing,in lieu of hydrogen, an isotope of hydrogen to passivate material in thevicinty of the base-emitter junction in a bipolar transistor. Forpurposes of the present invention, “substantial concentration” isdefined as a concentration of at least 10¹⁶ cm³¹ ³ of isotopic hydrogen.

The foregoing has outlined, rather broadly, preferred and alternativefeatures of the present invention so that those who are skilled in theart may better understand the detailed description of the invention thatfollows. Additional features of the invention that form the subject ofthe claims of the invention are described below. Those who are skilledin the art should appreciate that they can readily use the disclosedconception and specific embodiment as a basis for designing or modifyingother structures for carrying out the same purposes of the presentinvention. Those who are skilled in the art should also realize thatsuch equivalent constructions do not depart from the spirit and scope ofthe invention in its broadest form.

What is claimed is:
 1. A method of manufacturing a bipolar transistor,comprising: forming a base region, an emitter region and a base-emitterjunction between said base and emitter regions in a substrate; andintroducing a substantial concentration of an isotope of hydrogen insaid base-emitter junction.
 2. The method as recited in claim 1 whereinsaid isotope is deuterium.
 3. The method as recited in claim 1 furthercomprising forming a collector region and a base-collector junctionbetween said base and collector regions in said substrate.
 4. The methodas recited in claim 1 further comprising introducing a substantialconcentration of an isotope of hydrogen in said emitter region, whereinsaid emitter region is at least partially composed of polysilicon. 5.The method as recited in claim 1 further comprising forming a dielectriclayer over said substrate and introducing a substantial concentration ofan isotope of hydrogen in said dielectric layer.
 6. The method asrecited in claim 1 wherein said introducing comprises introducing asubstantial concentration of an hydrogen isotope into said emitterregion.
 7. The method as recited in claim 1 wherein a surface of saidsubstrate is generally planar, wherein said method comprisesconcentrating said isotope in portions of said planar surface of saidsubstrate.
 8. The method as recited in claim 1 wherein said substrate iscomposed at least partially of silicon.
 9. The method as recited inclaim 1 further comprising forming at least one electrical conductor tocarry electrical current to said transistor.